A valve having four switching positions and a mechanical locking arrangement using two balls is already known. If the user switches such a valve into the fourth switching position he will notice when actuating the control spool by means of a hand lever, that an increased actuating force is necessary to reach said fourth position. In this manner, the user is informed about the switching position. Another directional control valve is known wherein the fourth switching position can be maintained or fixed by means of a holding magnet. For said known directional control valves, particularly those with four switching positions, typically the fourth switching position, provides for the so-called floating position. With said known directional control valves there exists a disadvantage if said fourth switching position can be reached unknowingly or inadvertently. Such a situation can endanger people who are in the environment of an apparatus which is actuated by such a directional control valve.
Additional prior art is disclosed in the following German patent publications: 32 22 435, 12 40 354, 22 33 636, 21 64 728, 19 62 069, 17 50 510, and 22 13 244.
It is an object of the present invention to overcome the disadvantages of the prior art. It is a particular object of the present invention to provide a directional control valve, in particular a control valve having four switching positions such that it is made clear to the operator of the valve that a special coded position, e.g. said fourth switching position is being reached. It is another object of the present invention to provide an arrangement for avoiding the inadvertent switching of the valve into this special switching position in a relatively simple and inexpensive manner.
In accordance with the invention a directional control valve is provided having a control spool. A locking arrangement serves, to latch or lock the control spool in one of its switching positions, so that another switching position, the so called special switching position can only be reached if said locking arrangement is actuated by an additional signal which will release said control spool. Preferably, said additional signal is an electrical signal. It is further preferred to apply said signal only for a short period of time so as to allow the movement of the control spool of the valve from the initially locked or latched position into a position adjacent to said latched or locked position. Preferably the control spool of the directional control valve is spring centered. Additional preferred embodiments of the invention may be gathered from the dependent claims.